1. Field of the Invention
The present invention relates to a process for preparing purified calcium sulfate in an aqueous medium from natural or synthetic impure calcium sulfate.
The invention also relates to the polymorphous crystalline structures of purified calcium sulfate resulting from the process, without a controlled shape factor.
The invention also relates to the longilinear monomorphous crystalline structures of purified calcium sulfate resulting from the process, with a controlled length and shape factor.
2. Discussion of the Background
As used herein, the phrase "polymorphous crystalline structures" designates the field composed of individualized crystals which appear in the mixture both as needles with a very varied length and diameter, scales (twin crystals), polycrystalline aggregates and other crystalline structures.
The phrase "longilinear monomorphous crystalline structures" designates the field composed of individualized crystals which appear in the form of needles, or indeed composed of said crystals connected in threes at most along their large axis by partial covering, with a controlled length and shape factor.
Finally, the phrase "shape factor" is intended to define the size ratio existing between the length of the individual crystal or crystals connected in threes at most, and their largest diameter.
For a long time, dihydrated calcium sulfate, in the chemical form more commonly known by the name "gypsum", with the formula CaSO.sub.4, 2H.sub.2 O, has been the source of raw material for the production of plasters ("alpha" or "beta" varieties of semihydrated calcium sulfate commonly known by the name "hemihydrate", with the formula CaSO.sub.4 1/2 H.sub.2 O) resulting from its heat treatment (dry or in the presence of water in liquid or vapor form) between 90.degree. C. and 250.degree. C.; and for the production of anhydrites, I, II and III (with the formula CaSO.sub.4), with one being called soluble (anhydrite III) when the treatment is carried out at a temperature selected within the range of 100.degree. C. to 250.degree. C., and the others being called insoluble when the heat treatment exceeds the temperature of 250.degree. C. (anyhydrite II or overburned) or 1200.degree. C. (anhydrite I).
The gypsum used for these heat transformations can be natural, in which case it is removed from fossil deposits, or indeed of synthetic origin and, consequently, it can be phosphogypsum, desulfogypsum or gypsum from chemical neutralization in certain industrial processes.
Whether it is of natural or synthetic origin, the gypsum contains impurities which are more or less of a nuisance and which are perpetuated in the various chemical forms resulting from its heat transformation.
The most common impurities are, for example, organic compounds which as humic acids, simple inorganic compounds, such as halogenides, sulfates, calcium carbonate, magnesium carbonate, strontium carbonate, radium carbonate (and other carbonates), and also complex inorganic compounds such as clays, sodium fluosilicate, aluminum fluosilicate and calcium fluosilicate (A.V. SLACK, Fertilizer Science and Technology Series--Volume 2, Phosphoric acid, part 2, Marcel DEKKER, Inc., New York, 1986, pp. 531 and 643; K KAJI and T. TSUDA, Sekko to Sekkai, N. 48, 1960, p. 162-Ref.: Chemical Abstracts, Volume 55, 1961, 9830)), or even of the Ca.sub.4 AlSiSO.sub.4 F.sub.12 OH.multidot.12H.sub.2 O or Ca.sub.4 AlSiSO.sub.4 F.sub.13 .multidot.12H.sub.2 O types (L.P. YERMILOVA, V.A. MOLEVA and R.V. FLEVTSOVA, ZAl; Vsesoyuzn. Miner. Obshsh., 89, 15 (1960, Russian); J.R. LEHR, A.W. FRAZIER and J.P. SMITH, J. Agric. and Fd. Chem., 14, 27 (1966)), radioactive components, P.sub.2 O.sub.5 and fluorophosphates FPO.sub.3.sup.-). These impurities are for the most part insoluble or minimally soluble in water and thus require large amounts of water to dissolve them and remove them in this form.
Moreover, the presence of at least certain impurities in numerous natural or synthetic gypsums compromises the use of said gypsums as a source of raw materials in heat transformation processes, because these impurities can be at the origin of numerous harmful effects, for example on the setting kinetics of the plaster, on the lack of whiteness of the various chemical and crystalline forms of calcium sulfate (through heat transformation of the gypsum) when these various forms are intended to play a role of white pigmentary changes, or even on the appearance of the crystalline structures themselves, the control of the morphology and/or of the shape factor of which can be upset or even irreversibly compromised, and finally on the polluting and possibly troublesome aspect of radioactivity.
All of the interest in such a source of raw material is perceived as soon as its non-use appears as a large economic loss, both for natural and for synthetic gypsum, and as a major risk of pollution, since synthetic gypsums are presently stocked in thousands of tons in quarries and dumps or are dumped into streams and/or rivers, with rejects into running water becoming unacceptable due to the risks of eutrophization of said water.
This is why impure gypsums have been, as shown by the specialized literature, at the root of numerous descriptions of methods for purification and transformation into saleable products, with each process seeking to make its contribution to the fight against pollution, to the valorization of impure gypsums and to the control of the conditions for the removal of a certain impurity or of certain impurities.
A first type of purification process proposes removing the impurities from a source of hemihydrated calcium sulfate by solubilizing them in a chemically neutral liquid phase, through washing with spring water, following by a rapid separation of the washing liquor and almost immediate use of the purified hemihydrated calcium sulfate. Such a process, which is intended for the production of a purified hemihydrated calcium sulfate and which is illustrated by French Patent 2,359,692, consists of rapidly washing at a low temperature (between 0.degree. C. and 30.degree. C.) impure hemihydrated calcium sulfate coming from the heat transformation of a natural gypsum containing water-soluble impurities such as chloride, calcium sulfate and magnesium sulfate.
However, while this first type of method is intended to provide purified hemihydrated calcium sulfate, it has the major disadvantages of being able to be used only on hemihydrated calcium sulfates in which the impurities are water-soluble.
A second type of purification process proposes, as in the first type, using impure hemihydrated calcium sulfate and is distinguished from the preceding process by the essential fact that it apparently seeks to solubilize, in a strongly acidified aqueous phase, the impurities which are claimed to be insoluble in water.
A process of this second type is described in French Patent 2,064,195. It consists of treating the impure hemihydrated calcium sulfate, in a highly concentrated aqueous suspension (50 grams per liter to 650 grams per liter), with an acid agent by adjusting the pH of said suspension to a value preferably lower than 1.5 (to cause the impurities to dissolve), of maintaining the temperature of the suspension at a level selected within the range of at least 5.degree. C. to 60.degree. C., with the treatment being carried out in the presence of at least one organic agent, and of carrying out a hydroseparation of the solid phase at the end of the treatment.
A process of this type has disadvantages which can render its use unrealistic and raising doubts as to its industrial character, because the acid treatment of the aqueous suspension which is highly concentrated in impure material:
can achieve only a selective dissolution of the impurities since it yields a product having a degree of whiteness of 90.5 percent at best (Example 2), PA0 causes the rapid saturation in dissolved impurities of the liquid phase and the risk of reprecipitation of said impurities into the solid phase, PA0 requires a subsequent washing of the purified solid phase to remove the acid impregnation mother waters therefrom, PA0 is a source of environmental pollution due to the inevitable rejection of the liquid acid phase which is saturated with impurities, washing waters and organic liquids such as hydrocarbons, PA0 is economically disadvantageous due to its high consumption of water, acid reagents and possibly reagents for neutralizing the liquid effluents. PA0 a) the formation of an aqueous solution by dissolving impure hemihydrated calcium sulfate at a concentration at most of 13.0 grams per liter (expressed in dissolved calcium sulfate) and at a pH of at least 5.5, PA0 b) the separation of the aqueous solution containing the dissolved calcium sulfate from the insoluble solid phase formed by the impurities to be removed, PA0 c) the recrystallization of the purified calcium sulfate in dihydrated form from the aqueous solution resulting from step (b), possibly in the presence of a seeding primer introduced into the reaction medium, PA0 d) the separation, after recrystallization, of the aqueous phase containing a reduced amount of dissolved calcium sulfate from the solid phase composed by the recrystallized purified calcium sulfate, and PA0 e) the recycling of the aqueous phase containing a reduced amount of dissolved calcium sulfate back to step (a) for dissolving impure calcium sulfate.
A third type of purification process, which is distinguished from the other types, consists of solubilizing the impure gypsum, in a hot acid solution in which the impurities are, for the most part, insoluble, then of separating the impurities and of recrystallizing the purified gypsum by cooling of the calcium sulfate-rich solution.
A process of this last type is described in U.S. Pat. No. 3,642,456 and comprises the successive steps of dissolving the impure gypsum in a solution of fluosilicic acid (at a concentration of 15 percent to 26 percent of H.sub.2 SiF.sub.6) brought to a temperature of 70.degree. C. to 90.degree. C., separating at the same temperature the liquid phase and the solid phase composed of the insoluble impurities, cooling the liquid phase to a temperature of approximately 27.degree. C. to 33.degree. C. to cause the precipitation of the purified gypsum, and finally washing said gypsum to remove the acid solution therefrom.
However, this process, like the preceding ones, has certain disadvantages which render it difficult to use industrially.
Firstly, the hot aqueous solutions of fluosilicic acid used the process can cause serious corrosion phenomena which would require, in order to combat them, complex and very expensive installations.
In addition, the hot aqueous solutions dissolve, at least in part, the impurities which cocrystallize with the purified gypsum during the required cooling of the recrystallization solution.
Moreover, the aqueous acid solutions become a source of environmental pollution since they must be removed by a subsequent washing of the recrystallized purified gypsum which they impregnate.
Finally, the process appears to have no economic interest due to its high consumption of washing water which must be replaced, of acid reagents and possibly of reagents for neutralizing the liquid effluents, and due to its high energy consumption which is related to the consequent temperature variations used.
Therefore, although the prior art has recommended, through numerous publications, means to be used for preparing purified calcium sulfate from various sources of impure calcium sulfate, these means have been shown to be difficult to use on an industrial scale (process of type I), strong environmental polluters, large consumers of water, acids and neutralizing agents, and frequently to have little effect since they select certain impurities at the origin of the precipitation and/or the cocrystallization of the impurities which are initially solubilized with the purified calcium sulfate (processes of types II and III).
For this reason, it appeared necessary for the field of purification of calcium sulfate to have effective, economical and non-polluting means.